1. Field of the Invention
This invention relates to a high speed transistor switching circuit which prevents secondary breakdown in a power transistor and provides galvanic isolation between the transistor switching logic and the transistor itself.
2. Description of the Background Art
Power transistors have been used extensively to switch high voltages and currents. Little, if any, stress is placed on a transistor when switched from an off state to an on state by the application of base current causing the transistor to conduct load current. However, when a transistor, conducting high load currents during the on state, is switched to the off state, secondary breakdown in the transistor can occur. To switch the transistor off, the base current is reversed in polarity to sweep out charge carries from the base region of the transistor. After the base current reverses polarity, the base-emitter junction becomes increasingly negatively charged, causing focusing of all the collector current into a narrow ribbon in the center of the emitter active area. The focusing of the collector current in the emitter active area causes a secondary breakdown in the power transistor.
Known snubber circuits have been employed to momentarily bypass the load current when the transistor is switched from on to off states in order to prevent secondary breakdown in the transistor. Such snubber circuits typically include a parallel combination of a diode and a resistor connected in series with a capacitor across the collector and emitter of the power transistor. When the base current reverses polarity, the external snubber circuit will bypass some of the load current so that the overall collector current is safely below the breakdown rating of the transistor.
These known snubber circuits have a number of disadvantages. When switching of the transistor from the on to the off states occurs, the snubber circuit dissipates power and at high switching frequencies, this circuit consumes a large portion of the available power. Further, when used in conjunction with transistors in a bridge circuit, the snubber circuit associated with one transistor will cause large current transients to be produced in the complementary transistor of the bridge. In order to prevent transistor failure due to the large current transient caused by the snubber circuits, additional circuitry such as chokes and voltage clamps is required. The additional circuitry required is expensive, bulky in size and often results in additional problems.